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Re: Xenon Ion

Speak of the devil.


    A new NASA spacecraft engine that begins flight at less 
than a snail's pace but builds up enough speed to catch a 
comet will soon be used to push exploring spacecraft to the 
far reaches of the solar system.

     A prototype of a xenon ion engine, which fires 
electrically-charged atoms from its thruster, began a nearly 
year-long endurance test April 30 at NASA's Jet Propulsion 
Laboratory, Pasadena, CA.

    Once validated by the test, a similar engine will power 
the first New Millennium mission, called Deep Space-1, to an 
asteroid and a comet in 1998.  The comet will be West-
Kohoutek-Ikemura and the asteroid will be McAuliffe, named 
after the school teacher Christa McAuliffe who died in the 
Challenger accident.

    "NASA has been experimenting with ion drive engines for 
30 years," said Jack Stocky, manager of the ion propulsion 
system project.  "However, this test will be the most 
extensively instrumented endurance test of an ion engine ever 

    In space, the 11.8-inch diameter engine will use the 
heavy but inert xenon gas as fuel and be powered by more than 
2,000 watts from large solar arrays provided by the Ballistic 
Missile Defense Organization.  The actual thrust comes from 
accelerating and expelling the positively-charged atoms, 
called ions.  The thrusting action is similar to that of 
chemical propellant engines which expel burning gases, except 
that such engines can produce up to millions of pounds of 
thrust.  The engines in rockets that lift the Space Shuttle, 
for instance, combine metal-warping heat with an Earth-
shaking roar and quickly lift the Shuttle to more than 17,000 
miles per hour.

    An ion engine, however, starts with only about 20-
thousandths of a pound of thrust.  There's no roar, just an 
eerie blue glow.  While the atoms, charged by an electric arc 
which removes one of the 54 electrons around its nucleus, are 
fired in great numbers out the thruster at more than 70,000 
miles an hour, their accumulative mass is so low, the 
spacecraft moves only millimeters per second in its early 
stages of flight.

    Still, ion propulsion is more propellant efficient than 
chemical propulsion because it expels molecules from the 
engine at a much higher speed, Stocky said.  A chemical 
propulsion engine has an exhaust velocity of 10,400 miles per 
hour while ion propulsion exhaust is 70,200 miles per hour.

    Built at NASA's Lewis Research Center, Cleveland, OH, the 
engine will be tested for 8,000 hours (330 days) in the 
space-like environment of JPL's vacuum chamber.  "Ion engines 
have such low thrust they cannot operate in an atmosphere and 
have to be tested in a vacuum," said Dr. John Brophy, user 
validation assessment manager  for the project.  "JPL has the 
technical expertise and the cost-effective facility for the 
test."  The test is designed to run full power for two days 
and then shut off for one hour and restart.  This stressing 
process will be repeated until 8,000 hours of operation have 
been accumulated.

    After Deep Space-1 is launched by an expendable rocket 
with sufficient power to escape Earth's gravity, it will be 
in orbit around the Sun moving at the same speed the Earth 
moves in its orbit.  That means that relative to Earth, the 
spacecraft will not be moving at all.  But slowly, the low-
thrust ion engine will increase and the spacecraft's velocity 
over time to greet its celestial target at more than 22,000 
miles per hour, fast enough to rendezvous with a comet or 
asteroid.  The prototype ion engine carries 176 pounds of 
xenon in a tank, which in flight would last from one to two 
years, depending on its destination and the amount of total 
thrusting required, Brophy said.  Deep Space-1 will consume 
only 99 pounds of xenon during its mission.